The present invention relates to methods and apparatus for estimating a physiological parameter using Fourier transforms. More specifically, the invention relates to a pulse oximetry system for estimating the oxygen saturation of hemoglobin in arterial blood in which a saturation value is determined from representations of the oximeter sensor signals in a transformed space.
Pulse oximeters measure and display various blood flow characteristics and blood constituents including but not limited to the oxygen saturation of hemoglobin in arterial blood. An oximeter sensor passes light through blood-perfused tissue and photoelectrically senses the absorption of the light by the tissue. The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent being measured. The amount of light absorbed is then used to calculate the amount of the blood constituent present in the blood.
The sensed light signals can be degraded by both noise and motion artifact. One source of noise is ambient light that reaches the sensor's light detector. Another source of noise is electromagnetic coupling from other electronic instruments. Motion of the patient also introduces noise and affects the detected light energy. For example, the contact between the sensor's detector and/or emitter and the tissue sample can be temporarily disrupted when motion causes either to move away from the tissue. In addition, because blood is fluid, it responds differently than the surrounding tissue to inertial effects, thus resulting in momentary changes in volume at the point to which the oximeter sensor is attached. The degradation of the detected light energy can, in turn, result in degradation of the pulse oximeter output and inaccurate reporting of the blood constituent concentration. It will be understood that such inaccuracies can have negative consequences.
A variety of techniques have been developed to minimize the effects of noise and motion artifact in pulse oximetry systems. In a system described in U.S. Pat. No. 5,025,791, an accelerometer is used in the oximetry sensor to detect motion. When motion is detected, data taken during the motion are either eliminated or indicated as being corrupted. In U.S. Pat. No. 4,802,486, assigned to Nellcor Puritan Bennett, the assignee of the present invention, the entire disclosure of which is incorporated herein by reference, an EKG signal is monitored and correlated to the oximeter reading to provide synchronization to limit the effect of noise and motion artifact pulses on the oximeter readings. This reduces the chance of the oximeter locking onto a motion signal. In U.S. Pat. No. 5,078,136, assigned to Nellcor Puritan Bennett, the assignee of the present invention, the entire disclosure of which is incorporated herein by reference, signal processing techniques such as linear interpolation and rate of change analysis are employed to limit the effects of noise and motion artifact.
In another oximetry system described in U.S. Pat. No. 5,490,505, an adaptive noise canceler is used on different additive combinations of the red and infrared signals from the oximeter sensor to identify a coefficient for which the output of the noise canceler best represents the oxygen saturation of hemoglobin in the patient's blood. Unfortunately, this technique is computationally intensive resulting in an expensive implementation with undesirably high power requirements.
In yet another oximetry apparatus in U.S. Pat. No. 5,632,272, a technique using a Fourier transform is described. Data from the Fourier transform is analyzed to determine the arterial blood saturation, by considering all Fourier energies above a threshold with equal importance. However, the technique described in U.S. Pat. No. 5,632,272 is inadequate in the presence of significant random motion, where many anomalous signals exist above the noise threshold.
Because each of the above-described techniques has its own limitations and drawbacks, it is desirable to develop techniques for processing the signals from oximetry sensors to more accurately determine blood-oxygen levels in the presence of noise and motion artifact.